Lazy manufacturers. ALl that has to be added for a GPU to be Mac Compatible is an EFI capable ROM. After that, if you get some drivers, it's done. I find it funny that AMD is making mac compatible cards when Macs don't have AMD CPU's and likely never will.
You don't know history very well. AMD chips will always be an option for OS X and Apple, especially if Intel can't match growth and intentionally sandbags any future projects.